#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
using namespace std;
#include<vector>
int Find(vector<int>& v)
{
	int sum1 = 0, sum2 = 0;
	int n = v.size();
	int i = 0;
	while ( i < n)
	{
		sum1 += v[i++];
	}
	for (i = 0; i <= n; i++)
		sum2 += i;
	return sum2 - sum1;
}
int main()
{
	vector<int> v;
	v.push_back(1);
	v.push_back(2);
	v.push_back(3);
	v.push_back(0);
	int ret = Find(v);
	return 0;
}